Jackie Robinson wore uniform number 42 throughout his Major League Baseball career. He wore it with the Brooklyn Dodgers from his debut on April 15, 1947, through his final season in 1956.
The number became inseparable from Robinson because he wore it while breaking modern Major League Baseball’s color barrier. His career lasted ten seasons and included the 1947 Rookie of the Year Award, the 1949 National League MVP Award, six All-Star selections, and a 1955 World Series championship.
MLB retired number 42 across every team on April 15, 1997, the 50th anniversary of Robinson’s debut. Players already wearing it were allowed to keep it under a grandfather clause; Mariano Rivera was the last regular MLB player to wear 42, retiring after the 2013 season.
A related detail can cause confusion: players and umpires wear 42 on Jackie Robinson Day, but the number remains otherwise retired. Robinson also wore other numbers at UCLA, so the question specifically refers to his MLB career.